Solid state vacuum tubes

29 November 2017

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Paul McGowan writes:Most of us like the sound of vacuum tubes on an amplifier’s input: sweet, rich, full-bodied, musical. An equal number aren’t too happy about their downsides: heat, microphonics, size, and the 300-pound gorilla—their ephemeral lifespan. Which is why most customers rejoiced when we began to leverage a solid state equivalent, the FET.

Vacuum tubes are voltage controlled devices while bipolar transistors rely upon current. The two are very different sounding technologies. FETs, on the other hand, are very much like tubes in the way they require almost no current from the incoming signal. Not surprisingly, FETs—in particular, JFETs (and MOSFETS)—sound much closer to tubes than the more conventional bipolar transistors.

One of my first experiences with JFETs happened more than 30 years ago. In the mid-1980s designer Bob Odell had joined PS Audio’s design team. He had brought with him a new power amplifier design that would become the famous 200C—but not before we made some major changes to its input stage. The original design relied upon a traditional bipolar differential pair at the amp’s input. This worked well but the amp still had a slightly cold and transistory tinge to its musical character, one we wanted to eliminate before releasing it as our premier offering for power amplifiers. We decided to replace the bipolars with JFETs to see if this might help. Of course, rarely is it a drop in replacement (that’d be too easy). The JFETs of those days were limited in voltage requiring us to add a cascode circuit on the top of the new diff pair. It worked.

By replacing the input bipolars with a new type of transistor, the JFET, we were able to help the 200C amplifier get closer to the sound of vacuum tubes and honor the music. Nearly all PS amplifiers following the venerable 200C have enjoyed the benefits of either FETs or vacuum tubes at their input. And now you know why.

Vacuum tubes still outperform FETs on the input circuit of an amplifier, but FETs are a close second and without the PITA characteristics of tubes.If you’d like to learn a bit more about the differences between FETs and bipolars, click here to watch my latest video.

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